Flash-erasable semiconductor memory device having an improved reliability

ABSTRACT

A flash-erasable semiconductor memory device comprises a memory cell array including a plurality of memory cell transistors each having an insulated floating gate for storing information and a control electrode provided on said floating gate, wherein the flash-erasable semiconductor memory device includes a write control circuit supplied with a write control signal, when writing information. The write control circuit produces a control signal such that a leading edge of the drain control signal appears after a leading edge of the gate control signal. Further, the gate control circuit shuts off the gate control signal such that a trailing edge of the gate control signal appears after a trailing edge of the drain control signal.

BACKGROUND OF THE INVENTION

The present invention generally relates to semiconductor devices andmore particularly to a flash-erasable EPROM device or simply a flashmemory device that has an improved reliability.

In relation to the storage device of computers, there is a continuousdemand for a non-volatile semiconductor memory device having a largecapacity for storing information. Particularly, the so-calledflash-erasable memory device or simply flash memory device has beenstudied intensively in recent years as an alternative of hard diskdevices. In flash memory devices, rewriting of data is possiblesimilarly to the conventional random access memories, while the devicecan hold the written information even when the electrical power isturned off. Thus, the device is ideal for external storage device ofcomputers such as a hard disk. Further, application to the memory cardsis studied. In relation-to various applications of the flash memorydevice, intensive efforts are in progress to improve the reliability ofthe device.

FIG.1 shows the structure of a typical memory cell transistor that formsa flash memory device.

Referring to FIG. 1, the memory cell transistor is constructed on asemiconductor substrate 1 and includes a source region 6 and a drainregion 7 formed in the substrate 1 similarly to a conventional MOStransistor. On the upper major surface of the substrate 1, a gateinsulation film 2 is provided to cover the channel region extendingbetween the source region 6 and the drain region 7, and a gate electrode3 is formed on the gate insulation film 2 in correspondence to thechannel region of the device. Further, a capacitor insulation film 4 isprovided to surround the gate electrode 3, and the gate electrode 3thereby forms a floating gate electrode. Further, an electrode 5 isprovided on the floating gate electrode 3 such that the electrode S isinsulated from the electrode 3 by the capacitor insulation film 4.Thereby, the electrode 5 is used as a control electrode.

Hereinafter, the operation of a flash memory of the NOR type will bedescribed.

When writing data, a predetermined drive voltage is applied across thesource region 6 and the drain region 7 such that the electrons arecaused to flow from the source region 6 to the drain region 7.Simultaneously, a large positive voltage is applied to the controlelectrode 5 to induce a large electric field between the floating gateelectrode 3 and the substrate 1. Thereby, the electrons transportedalong the channel region and accelerated in the vicinity of the drainregion 7 are injected into the floating gate electrode 3 through thegate insulation film 2 as hot electrons, Once the electrons areinjected, the electric charges associated with the electrons controlsthe conduction of the channel region between the source and drainregions 6 and 7. In other words, one can read the content of the datawritten into the memory cell transistor by detecting the conductionthereof. when erasing data, on the other hand, a large electric field isinduced between the floating gate 3 and the source region 6 by applyinga large positive voltage to the source region 6. Thereby, the electronsin the floating gate 3 dissipate into the source region 6 by causing atunneling through the gate insulation film 2.

FIG.2 shows the foregoing control scheme of the flash memory device forthe writing mode for wiring data into the memory cell, the reading modefor reading data from the memory cell, and the erasing mode for erasingdata from the memory cell, wherein the voltage V_(H) is set typically to+12 volts. while the voltage V_(M) may be set to +6 volts. Further, thevoltage V_(L) is set to about +5 volts.

FIG. 3 shows the overall construction of a typical flash memory device.

Referring to FIG. 3, the device includes a memory cell array 11 in whicha plurality of memory cell transistors each having a construction ofFIG. 1 are arranged in rows and columns, and the memory cell in thememory cell array 11 is selected in response to address data that issupplied to a row address buffer circuit 12 for activating a row decoder13 and a column address buffer circuit 14 for activating a columndecoder 15. There, the row decoder 13 selects a word line WL in responseto the row address data latched in the row address buffer circuit 12while the column decoder 15 controls a column selection gate 16 toselect a bit line BL in response to the column address data that islatched in the column address buffer circuit 14.

In order to achieve inputting and outputting of data, there is provideda data bus 17 connected to an input/output buffer circuit 18, and thedata on the bus 17 is written into a selected memory cell such as thememory cell 11a via a write amplifier 19 and the column selection gate16. On the other hand, the data stored in the selected memory cell istransferred to the input/output buffer circuit 18 via the columnselection gate 16 and a sense amplifier 20. Further, in order to controlthe read/write operation of the memory device, there is provided anotherbuffer circuit 21 that is supplied with an output enable signal /OE, achip enable signal /CE, and further with a write enable signal /WE,wherein the signal /OE is used for enabling the data output of theinput/output buffer circuit 18, the signal /CE is used for the chipselection, and the signal /WE is used for enabling the writing of datainto the selected memory cell.

Further, there is provided an erase power supply unit 22 thatcharacterizes the NOR type flash memory device, wherein the power supplyunit 22 supplies a predetermined erase voltage when erasing the datafrom the memory cell array. As is well known, the erasing of data occurssimultaneously for all the memory cells in the memory cell array 11 inthe flash memory device. In addition, in order to control the operationof memory cell device including the erase power supply 22, a controller23 is provided. There, the controller 23 is supplied with data from thedata bus 17 as well as an output of the buffer circuit 21 and controlsthe read/write as well as erase operation of the device.

FIG. 4 shows the writing of data into the memory cell transistor of FIG.1, wherein the vertical axis represents the drain current and thehorizontal axis represents the drain voltage. As already noted withreference to FIG. 2, the voltage V_(D) is applied to the drain regionduring the writing process of data, while the voltage V_(H) is appliedsimultaneously to the control gate.

Referring to FIG. 4, it will be noted that the drain current increasesin an interval designated as "1" with increasing drain voltage V_(D),while the drain current decreases suddenly in correspondence to theinterval designated as "2" with further increase in the drain voltageV_(D). In correspondence to this negative bump of the drain current, theinjection of electrons into the floating gate electrode occurs. Further,when the drain voltage V_(D) has reached an avalanche voltage V_(ABD),an avalanche breakdown occurs in the channel region of the memory celltransistor and the drain current increases steeply. Thereby, anefficient injection of the electrons is achieved into the floating gate.Thus, the flash memory device generally uses the avalanche voltageV_(ABD) for the voltage V_(M) shown in FIG. 2 to achieve an efficientwriting of the data. In fact, the drain voltage V_(D) is clamped at thelevel V_(ABD) when the foregoing positive control voltage V_(H) isapplied to the control gate.

On the other hand, when the voltage of the control gate is low or zeroin correspondence to an operational state of the device wherein nowriting of data occurs, the drain current changes as shown in the brokenline in FIG. 4. There, the drain current remains low until a breakdownvoltage V_(JCT) is reached. In response to the voltage V_(JCT), abreakdown occurs at the p-n junction between the drain region and thesubstrate. Generally, the voltage V_(JCT) is larger than V_(ABD) by morethan one volt. Thereby, there can occur a possibility that the writingof data into a first memory cell transistor can affect the operation ofa second memory transistor that shares the power supply line commonlywith the memory cell transistor. It should be noted that the large drainvoltage applied to the first memory cell transistor for writing datainduces a large electric field between the drain region and the floatinggate in the second memory cell transistor. Thereby, the electric chargesstored in the second memory cell transistor can dissipate into the drainof the same memory cell transistor and the data held therein isdestroyed. This interference of memory cell transistors is known asdisturbance."

Further, the conventional flash memory device has suffered from theproblem of limited flexibility in the design of redundant constructionin that only the column redundancy is possible as shown in FIG. 5.

Referring to FIG. 5, the drawing corresponds to FIG. 3 and includes thememory cell array 11 that in turn includes a number of memory cells M₁,1-M₂,3 provided in correspondence to intersections of word lines WL₁ -WL₂and bit lines BL₁ -BL₃. In FIG. 5, those parts corresponding to theparts described previously are designated by the same reference numeralsand the description will be omitted. It will be noted that the columnselection gate 16 includes transfer gate transistors Tsw₁ -Tsw₃ forselecting the bit lines BL₁ -BL₃ respectively.

In the memory cell array 11 of FIG. 5, it will be noted that there isprovided another transfer gate transistor Tsw₄ that is activated inresponse to an output of a decoder 24 for selecting another bit lineBL₄, and a column redundant memory cell array 11_(CR) is provided inconnection to the bit line BL₄. There, the redundant memory cell array11_(CR) includes memory cell transistors M₁,4 and M₂,4 having respectivedrains connected commonly to the bit line BL₄ and the memory cell array11_(CR) is activated in response to the output of the redundant decoder24 that in turn is controlled by a defect detection circuit 25. There,the circuit 25 is supplied with the column address data from a columnbuffer circuit 14 and compares the same with the address data fordefective memory cells stored in a memory device not illustrated. Whenthe address data indicates the selection of a defective memory cell, thecircuit 25 activates the redundant decoder 24 that in turn selects theredundant bit line BL₄. It should be noted that such a redundant memorycell array may be provided as a part of a utility memory cell array thatis provided separately from the real memory cell array for variouspurposes such as testing. In other words, one can use such a memory cellarray 11_(CR) also for testing as will be discussed later with referenceto the embodiment of the present invention.

In such a conventional flash memory device, it is desirable to provide arow redundant memory cell array in addition to the column redundantmemory cell array 11_(CR) for increasing the degree of freedom forsaving the defect in the memory cells. However, such a construction ofrow redundant memory cell array has been generally impossible in theflash memory devices. Hereinafter, the reason of this undesirablesituation will be examined briefly.

In the flash memory devices, the electric charges are removed from thefloating gate each time the data stored in the memory cell is erased. Asalready noted, such an erasing process is conducted by applying apositive voltage to the source region. Thereby, all the memory cellsthat are connected commonly to the source supply voltage experiencedissipation of the electric charges from the floating gate. In otherwords, the data stored in the memory cells that form the memory cellarray of the device are erased simultaneously.

Another point that requires special attention in the flash memorydevices is that the dissipation of electrons from the floating gateshould be achieved in such a manner that no substantial electric chargesremain in the floating gate after the erasing of data has occurred inthe memory cell. When the removal of electrons is excessive, thefloating gate may charge positively and the memory cell transistor isturned on permanently. In order to avoid this problem of "excessiveerasing," it is generally practiced to write data "0" into the memorycell by injecting electrons to the floating gate before each erasingprocess of data.

Thus, when a row redundant memory cell array is constructed by modifyingthe circuit of FIG. 5, for example such that the word line WL₂ isselected in place of the word line WL₁ for saving defective memory cellsconnected to the word line WL₁, the writing of the data "0" does notoccur to the memory cells connected to the word line WL₁. On the otherhand, the removal of the electric charges occurs also in these memorycells in response to the erasing process, as these memory cells areconnected also to the erase power supply unit 22. Thereby, the memorycell transistors M₁,1 -M₁,4 connected to the word line WL₁ areinevitably erased excessively as a result of the excessive removal ofthe electrons to the drain region. When this occurs, the floating gateis injected with holes and the memory cell transistors take apermanently turned-on state. As the transistors M₁,1 -M₁,4 are connectedto the bit lines BL₁ -BL₄, such an erroneous turning-on of the memorycell transistors inevitably causes a erroneous voltage level of the bitlines and the overall operation of the flash memory device becomesdefective.

In the conventional flash memory devices having the column redundancy asshown in FIG. 5, it is proposed to divide the memory cell array into aplurality of blocks each driven by an independent power supply unit suchthat the simultaneous erasing of data occurs only in each block insteadof the entirety of the memory cell array. When the column redundancy isapplied to such a device, however, the redundant memory cell columns areprovided in each block and there arises an inconvenience in that asubstantial device area is occupied by the redundant memory cellcolumns. Thereby, there is a demand to reduce the area of the devicethat is occupied by the redundant memory cell column.

In the conventional memory devices such as dynamic random accessmemories or static random access memories, it has been practiced toprovide a utility memory cell block for testing the device. Such autility block is used for example for the purpose of guaranteeing apredetermined number of times for the rewriting of data into the memorycell transistors forming the memory cell array. In the flash memorydevices, however, erasing of data is achieved in the ordinary, "real"memory cell block each time the data is erased from the utility memorycell block, as long as the memory cell transistors in the real memorycell block share the electric power supply with the memory celltransistors in the utility memory cell block. Thereby, the memory celltransistors in the real memory cell block are erased excessively and theproper read/write operation of the device is no longer possible. Inother words, the conventional flash memory devices have suffered fromthe problem that the test for guaranteeing the number of times the writeoperation can be achieved properly is impossible.

In addition, in the conventional flash memory devices, there has been aproblem in that one has to design the device to have a relatively largechannel length in correspondence to the relatively large voltage appliedto the source region for erasing data from the memory cell transistors,such that a sufficient junction breakdown voltage is secured. On theother hand, such a large channel length inevitably imposes a problem inthe miniaturization of the device. Thus, it is desired to reduce themagnitude of the voltage that is applied to the memory cell transistorfor erasing information therefrom.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful flash memory device, wherein the foregoing problemsare eliminated.

Another and more specific object of the present invention is to providea flash memory device, wherein a reliable read/write operation isguaranteed by controlling the magnitude of the voltage applied to thecontrol electrode such that the control voltage is clamped at theavalanche voltage. Thereby, the problem of excessive rising of thecontrol voltage and hence the problem of the interference betweenadjacent memory cell transistors is avoided.

Another object of the present invention is to provide a flash memorydevice that includes a row-redundant construction. As a result of therow-redundancy, the degree of freedom for realizing the redundancy issubstantially increased.

Another object of the present invention is to provide a flash memorydevice having a column redundant construction, wherein the power supplyfor supplying electrical power to the real memory cell block and to theredundant memory cell block is improved.

Another object of the present invention is to provide a flash memorydevice having a utility memory cell block in addition to a real memorycell block, wherein the problem of excessive erasing in the real memorycell block during the operational interval of the utility memory cellblock is successfully eliminated.

Another object of the present invention is to provide a flash memorydevice having a column redundancy, wherein the device has a simplifiedconstruction for selecting the redundant memory cell block incorrespondence to the selection of a defective memory cell column.

Another object of the present invention is to provide a flash memorydevice wherein the supply voltage used for erasing data is reduced.

Other objects and further features of the present invention will becomeapparent from the following detailed description when read inconjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the structure and operation of a typicalflash memory cell transistor;

FIG. 2 is a diagram showing the biasing of the flash memory celltransistor for various operation of the memory device;

FIG. 3 is a circuit diagram showing an overall construction of a typicalconventional flash memory device;

FIG. 4 is a diagram showing an injection of electrons into a floatinggate of the flash memory;

FIG. 5 is a block diagram showing the structure of a conventional flashmemory having a column redundancy;

FIG. 6 is a diagram showing the principle of a first embodiment of thepresent invention;

FIG. 7 is a diagram showing the circuit diagram of a control circuitused in the flash memory of FIG. 6;

FIGS. 8(A)-8(C) are diagrams-showing the timing for biasing the flashmemory of FIG. 6;

FIG. 9 is a block diagram showing the overall construction of a flashmemory that uses the control circuit of FIG. 7;

FIG. 10 is a block diagram showing the construction of a flash memoryhaving a row redundancy according to a second embodiment of the presentinvention;

FIG. 11 is a diagram showing the biasing scheme used in the flash memoryof FIG. 10 for erasing information;

FIG. 12 is a diagram similar to FIG. 11 for showing an alternativebiasing scheme for erasing information in the second embodiment;

FIG. 13 is a circuit diagram showing the construction of a row buffercircuit used in the memory device of FIG. 12;

FIG. 14 is a circuit diagram showing the construction of a row decoderused in the memory device of FIG. 12;

FIG. 15 is a drive of the row decoder for producing different word linevoltages;

FIG. 16 is a block diagram showing the flash memory having a columnredundancy according to a third embodiment of the present invention;

FIG. 17 is a block diagram showing the essential part of the memorydevice of FIG. 16;

FIG. 18 is a circuit diagram showing a modification of the memory deviceof FIG. 16;

FIG. 19 is a circuit diagram showing a fourth embodiment of the memorydevice of FIG. 16;

FIG. 20 is a diagram showing the erasing characteristics of a typicalflash memory cell transistor;

FIG. 21 is a diagram showing the biasing condition that provides therelationship of FIG. 20;

FIGS. 22(A)-22(D) are diagrams showing the optimization of the currentsupplying capability in the embodiment of FIG. 19 in conformity with thenumber of memory cell transistors included in the memory cell array;

FIG. 23 is a block diagram showing the principle of a fifth embodimentof the present invention;

FIG. 24 is a block diagram showing the construction of the flash memoryaccording to the fifth embodiment of the present invention;

FIG. 25 is a circuit diagram showing an essential part of the memorydevice of FIG. 24;

FIG. 26 is a block diagram showing the construction of the flash memoryaccording to a sixth embodiment of the present invention;

FIG. 27 is a circuit diagram showing a part of the memory device of FIG.26;

FIG. 28 is a flowchart for showing a test process for rejectingdefective memory devices;

FIG. 29 is a block diagram showing the construction of the flash memoryaccording to a seventh embodiment of the present invention;

FIG. 30 is a circuit diagram showing an essential part of the memorydevice of FIG. 29;

FIG. 31 is a circuit diagram showing a part of the memory device or FIG.29;

FIG. 32 is a block diagram showing a modification of the flash memory orFIG. 29;

FIG. 33 is a circuit diagram showing a part of the flash memory of FIG.32;

FIG. 34 is a diagram showing the structure of a flash memory accordingto an eighth embodiment of the present invention;

FIG. 35 is a diagram showing a modification of the flash memory of FIG.34;

FIGS. 36(A)-36(F) are diagrams showing the fabrication process of thedevice of FIG. 34; and

FIG. 37 is a diagram showing the structure of a memory cell transistorused in the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 6 shows the principle of the first embodiment of the presentinvention.

Referring to FIG. 6, the flash memory of the present embodiment includesthe memory cell array schematically illustrated in the form of thememory cell transistor 11a, wherein the control circuit 23 of the flashmemory as represented schematically in FIG. 3 includes a signalgenerator 233 that is supplied with an output signal PGM of the buffercircuit 21 in response to the write enable signal /WE when writing data,There, the circuit 233 produces a first control signal PGMR and a secondcontrol signal PGMC in response to the signal PGM, wherein the signalPGM causes a transition to the high level state during the interval inwhich the writing of data is achieved. The circuit 231 in turn producesa gate voltage V_(G) in response to the signal PGMC and the voltagelevel of the control gate represented as "G" in FIG. 6 is held at a highlevel state corresponding to the voltage V_(H) during the interval inwhich the signal PGMC assumes the high level state. The circuit 232 inturn produces a drain voltage represented as V_(D) in FIG. 6 such thatthe drain voltage V_(D) is held at the level V_(M) during the high levelinterval of the signal PGMC.

In the present embodiment, in order to guarantee that the drain voltageV_(D) shown in FIG. 4 is clamped at the avalanche voltage V_(ABD), thegate voltage V_(G) is set to the high level state V_(H) before the drainvoltage V_(D) rises to the high level state V_(M), and the gate voltageV_(G) maintains the high level state even after the drain voltage V_(D)has caused a transition to the low level state. Thereby, the problem ofthe drain voltage V_(D) exceeding the voltage V_(ABD) is successfullyavoided and the problem of disturb or interference between the memorycell transistors is eliminated.

FIG. 7 shows the circuit diagram of the circuit 233.

Referring to FIG. 7, it will be noted that the circuit 233 includes aninverter 233a supplied with the signal PGM and produces an output as alogic inversion of the signal PGM. The output of the inverter 233a issupplied on the one hand to a NAND gate 233d and on the other hand to aNOR gate 233e, wherein the NAND gate 233d and the NOR gate 233e form aflip-flop circuit that includes a first feedback path for feeding backthe output of the NAND gate 233d to another input terminal of the NORgate 233e via an inverter 233b and a second feedback path for feedingback the output of the NOR gate 233e to another input terminal of theNAND gate 233d via an inverter 233c. Thereby, the output signals PGMRand PGMC are obtained respectively at the output of the NAND gate 233dand the output of the NOR gate 233e with a timing relationship as shownin FIGS. 8(A)-8(C), wherein FIG. 8(A) shows the waveform of the signalPGM, FIG. 8(B) shows the waveform of the signal PGMR, and FIG. 8(C)shows the waveform of the signal PGMC.

Referring to FIGS. 8(A)-8(C), it will be noted that PGMR risessubstantially in synchronization to the leading edge of the PGM, whilethe leading edge of the PGMC is delayed with respect to the leading edgeof the PGM by an interval td₁, wherein the interval td₁ corresponds tothe delay caused by the NAND gate 233d and the inverter 233b. In otherwords, the delay td₁ is set such that the rise of the drain voltageV_(D) occurs after the rise of the gate voltage V_(G) in the circuit 233of FIG. 7. Further, it should be noted that the PGMC causes a transitionto the low level state substantially in synchronization with thetrailing edge of the PGM, while the trailing edge of the PGMR apppearsafter a delay of td₂ with respect to the trailing edge of the PGM,wherein the delay td₂ is determined by the delay caused by the NOR gate233e and the inverter 233c. Again, the delay td₂ is determined such thatthe gate voltage returns after the drain voltage returns to the lowlevel state. This indicates that the gate voltage V_(G) is held at thehigh level state V_(H) for a while, even when the drain voltage V_(D)has caused a transition to the low level state. As a result of thetiming relationship shown in FIGS. 8(A)-8(C), it will be noted that inno moment the situation occurs wherein the drain voltage V_(D) riseswhile the gate voltage V_(G) is held at the low level state. Thereby,the voltage level V_(D) never rises beyond the avalanche voltageV_(ABD), and the problem of the disturbance described earlier iseffectively eliminated,

FIG. 9 is a block diagram similar to FIG. 5 and shows the constructionof the memory device that uses the foregoing circuit 233 for theformation of the signals PGMR and PGMC. In FIG. 9, those partscorresponding to FIG. 5 are designated by the same reference numeralsand the description will be omitted.

Referring to FIG. 9, it will be noted that the PGMR is supplied to a rowselection power supply circuit 13A that produces the output signal V_(G)in response to the high level state of the PGMR, and the signal V_(G) issupplied to the row decoder 13 to which the row address data is suppliedsimultaneously. There, the row decoder 13 selects a word line such asWL₁ in response to the row address data supplied thereto and suppliesthe gate voltage V_(G) to the selected word line WL₁. Further, there isprovided a column power supply circuit 15A that is supplied with thePGMC, wherein the circuit 15A activates the column decoder 15 inresponse to the high level interval of the PGMC. Further, the PGMC issupplied also to a drain power supply circuit 19A that forms a part ofthe write amplifier 20, and the drain voltage V_(D) is controlled asalready described with reference to FIGS. 8(A)-8(C) in response to thePGMC. As the chance that the gate voltage of the selected word line suchas WL₁ remains low while the level of the voltage V_(D) is held at thehigh level state V_(M) is positively eliminated in the presentconstruction, the drain voltage V_(D) never increases beyond theavalanche voltage V_(ABD), and the problem of the disturbance issuccessfully eliminated.

It should be noted that the delay times td₁ and td₂ are determined to belarger than the difference between the signal delay occurring in theconductor strip that transfers the signal PGMR from the circuit 233 tothe control gate of the selected memory cell transistor and the signaldelay occurring in the conductor strip for transferring the signal PGMCfrom the circuit 233 to the drain region of the selected memory celltransistor. It should be noted that the conductor strip used forcarrying the signal PGMR is generally formed of polysilicon and thedelay occurring in the conductor strip for transferring the signal PGMRis generally larger than the delay occurring in the conductor thattransfers the signal PGMC. By setting the delay timed td₁ and td₂ assuch, the phase relationship shown in FIGS. 8(B) and (C) is guaranteedalso on the control gate and on the drain region of the memory celltransistor.

Next, a second embodiment of the present invention will be describedwith reference to FIG. 10, wherein the present embodiment relates to theflash memory device having the row redundancy. In FIG. 10, those partscorresponding to the parts already described with reference to FIG. 5 orFIG. 9 are designated by the same reference numerals and the descriptionthereof will be omitted.

Referring to FIG. 10, the memory cell array 11 includes a row and columnformation of the real memory cell transistors M₁,1 -M₃,3 that areselected by the row decoder 13 and the column decoder 15 similarly tothe embodiment of FIG. 5, wherein it will be noted that there isprovided an additional memory cell block that includes memory celltransistors M₃,1 -M₃,3 also in the memory cell array 11 as a rowredundant memory cell block, and a defect detection circuit 34 fordetecting the selection of a defective word line and a redundant worddecoder 35 for selecting a redundant word line such as WL₃ in responseto the output of the defect detection circuit 34, are provided foractivating the row redundant memory cell block. Herein, the phrase"defective word line" means a word line to which a defective memory celltransistor is connected. In addition, there is provided an additional,utility memory cell block connected to a word line WL₄ that is selectedby a utility word decoder 36. The utility memory cell block includesmemory cell transistors M₄,1 -M₄,3 and is used for testing the operationof the flash memory device.

In the present embodiment, in order to avoid the problem of theexcessive erasing of information from non-selected memory cellsdescribed previously in relation to the problem of row redundancy in theflash memory devices as well as the problem of the erase disturbancephenomenon, the present embodiment employs a construction to apply alarge negative voltage -V_(E) selectively to the control electrode ofthe memory cell transistor from which the information is to be erased,such that a dissipation of the electrons occurs from the floating gateto the substrate as indicated in FIG. 11. There, a positive voltageV_(L) corresponding to the voltage V_(L) described with reference toFIG. 2 is applied to the substrate of the device. At the same time, azero or positive voltage is applied to the control electrode of thenon-selected memory cells to avoid dissipation of the electric chargesfrom the floating gate of the non-selected memory cells to thesubstrate. Typically, the voltage V_(E) is set to -9 volts. Further, thedissipation of the electrons may be caused from the floating gate to thesource region by applying the voltage V_(E) to the control gate and thevoltage V_(L) to the source simultaneously as indicated in FIG. 12. Bycontrolling the erasing operation of the flash memory device as such,the problems of the excessive erasing and the erase disturb associatedwith the row redundancy are effectively eliminated. In addition, such aconstruction enables the use of the utility memory cell block that maybe used for the testing the rewriting operation. Conventionally, such autility memory cell block could not be provided on the same chip of thememory cell device because of the problem of the excessive erasing.

In order to achieve the foregoing object, the present embodiment shownin FIG. 10 employs a construction of the row address buffer circuit 12as shown in FIG. 13, wherein only a part of the circuit is illustrated.It should be noted, on the other hand, that the power supply 22 is nolonger used for the erasing purpose. Thus, the power supply 22 merelyproduces zero volt or a voltage corresponding to the level V_(H).

Referring to FIG. 13, the circuit 12 includes a NOR gate 12a that issupplied on the one hand with a control signal PD that is set to the lowlevel state during the operational state of the flash memory device andon the other hand with an address signal included in the multiple-bitaddress data, wherein the output of the NOR gate 12a is supplied to afirst input terminal of another NOR gate 12c via an inverter 12b.Further, the output of the NOR gate 12a is supplied to an input terminalof a NOR gate 12d. There, both the NOR gate 12c and the NOR gate 12d aresupplied with an output of the defect detection circuit 34simultaneously, wherein the defect detection circuit 34 produces theoutput such that the output of the circuit 34 assumes a high level statewhen a selection of the defective word line is made. When thedefect-free word line is selected, on the other hand, the output of thecircuit 34 is of course set to the low level state. When the output ofthe circuit 34 assumes the high level state, it will be noted that boththe output signal AD of the NOR gate 12c and the output signal /AD ofthe NOR gate 12d are set to the low level state. It should be that thecircuit of FIG. 13 is provided in number corresponding to the number ofthe bits of the address data to form the address buffer circuit 12.

FIG. 14 shows the construction of the row decoder 13, wherein the rowdecoder 13 includes a NAND gate 13a that receives the address data fromthe row address buffer circuit 12. There, the output of the row decoder13 is supplied, via an n-channel MOS transistor 13b and a p-channel MOStransistor 13g both urged to a turned-on-state by the supply voltagesV_(cc) and V_(ss) respectively, to a latch circuit that includes ap-channel MOS transistor 13h and an n-channel MOS transistor 13cconnected in series between a power supply voltage V_(DD2) and anotherpower supply voltage V_(DS2). The latch circuit further includes ap-channel MOS transistor 13i and an n-channel MOS transistor 13dconnected in series between the foregoing power supply voltages V_(DD2)and V_(DS2), wherein the transistors 13h and 13i as well as thetransistors 13c and 13d are connected such that the voltage at the nodebetween the transistors 13h and 13c is supplied to the respective gatesof the transistors 13i and 13d and such that the voltage at the nodebetween the transistors 13i and 13d is supplied to the respective gatesof the transistors 13h and 13c. The output of the latch circuit isobtained at the node between the transistors 13i and 13d and is suppliedto the gate of an n-channel MOS transistor 33e that is connected inseries to another n-channel MOS transistor 13f, There, the transistors13e and 13f are connected in series between a supply voltage V_(DS1) anda supply voltage V_(DS2), wherein the transistor 13f is supplied withthe output of the transistor 13g directly. Thereby, the control voltageto be outputted on the word line WL_(i) (i=1, 2, . . . ) is obtained atthe node between the transistor 13e and the transistor 13f.

In the construction of FIG. 14, the voltage V_(cc) is typically set to+5 volts while the voltage V_(ss) is set typically to zero volt.Further, it should be noted that the voltages V_(DD1) and V_(DS2) areset, in the erasing mode, to the foregoing negative voltage V_(E) thatis applied to the control gate of the memory cell transistor for erasingdata therefrom. On the other hand, the voltages V_(DD2) and V_(DS1)correspond to the voltage V_(L) that is supplied to the substrate in theerasing scheme of FIG. 11. In the erasing mode shown in FIG. 12, thevoltage V_(L) is supplied to the source region. Anyway, it will be notedthat the circuit of FIG. 14 outputs the control voltage on the selectedword line WL_(i) such that the control voltage is set to the level V_(E)in the erasing modes It should be noted that the NAND gate 13a producesa low level output in response to the address data that selects the wordline WL_(i). On the other hand, when the word line is not selected, theNAND gate 13a produces a high level output and a control signal havingthe level V_(L) is outputted on the non-selected word line in responsethereto.

A similar construction is used for the row redundant word line decoder35 and the utility word line decoder 36. In the case of the rowredundant word line decoder 35, the NAND gate 13a is replaced by aninverter 13_(j) that is supplied with the output signal from the defectdetection circuit 34. Similarly, an inverter 13k is used in place of theNAND gate 13a in the utility word line decoder 36, wherein the inverter13k produces an output in response to a test signal supplied thereto.Thus, when the row redundant word line decoder 35 is activated inresponse to the selection of a defective word line, the redundant wordline WL₃ shown in FIG. 10 is selected and the voltage level of theredundant word line WL₃ is urged to the level V_(E) in correspondence tothe supply voltage V_(DD1) when erasing data. Simultaneously, the restof the word lines are all urged to the voltage V_(L) in correspondenceto the supply voltage V_(DS1). In other words, the flash erasingoperation applied to the row redundant memory cell block does not causethe adversary erasing in the real memory cell block. A similar argumentapplies also to the utility memory cell block connected to the word lineWL₄. There, the erasing operation applied to the utility memory cellblock for testing the device operation can be achieved independentlyfrom the rest of the memory cells and the problem of the excessiveerasing is eliminated.

In the foregoing construction of the row decoder 13 shown in FIG. 14, itwill be noted that one has to set the supply voltages V_(DD1) andV_(DD2) to another voltage level different from V_(E) in the writingmode and in the reading mode. As shown in FIG. 2, one has to set theword line voltage of the selected word line to a large positive levelV_(H) in the writing mode while in the reading mode, the word linevoltage has to be set to the level V_(L).

FIG. 15 shows a circuit 13X for causing the foregoing change of thesupply voltage V_(DD1) in response to the operational mode of the flashmemory device.

Referring to FIG. 15, the circuit 13X forms a part of the circuit 13 wewell as 35 and 36 in the illustration of FIG. 10 and includes ap-channel transistor 191 having a source connected to the supply voltageV_(cc) and a p-channel transistor 192 connected in series to thetransistor 191. Further, there is provided a p-channel transistor 193having a source connected to the supply voltage V_(pp) and anotherp-channel transistor connected in series to the transistor 193. Therespective sources of the transistors 192 and 195 are connected commonlyto another p-channel transistor 195 that is turned on in response to theerase control signal E. There, the transistors 191 and 192 haverespective gates supplied with the write control signal W while thetransistors 193 and 194 have respective gates supplied with thecomplementary write control signal /W.

Thus, when operated in the reading mode wherein the signal W is high,the transistors 191 and 192 are turned on while the transistors 193 and194 are turned off, and the supply voltage V_(cc) is supplied to thetransistor 195. In the mode other than erasing, the control signal Etakes a low level state and the supply voltage V_(cc) thus supplied viathe transistors 191 and 192 is supplied further through the transistor195 to the supply terminal V_(DD1) of the circuit 13 shown in FIG. 14.Similarly, when the signal W is high in correspondence to the writingmode, the transistors 193 and 194 are turned on while the transistor 191and 192 are turned off, and the high positive supply voltage V_(pp) issupplied to the terminal V_(DD1) via the transistor 195 that is turnedan in response to the non-erasing mode.

Further, the circuit of FIG. 15 includes a number of p-channeltransistors 196, 197, 198, . . . connected to form a diode wherein thetransistors 196, . . . are connected in series to the supply voltageV_(SS) via an h-channel transistor 195A that is turned on and turned offin response to the erase control signal E. In the non-erase mode, thetransistor 195A is turned off in response to the low level state of thecontrol signal E, and the transistors 196, 197, 198, . . . aredisconnected from the supply voltage V_(SS).

On the other hand, in the erasing mode, the signal E takes a high levelstate and the transistor 195 turns off while the transistor 195A isturned on. Thereby, the supply voltage V_(SS) is supplied after voltagedrop caused by the diode connection of the transistors 196, . . . and alarge negative voltage corresponding to V_(E) appears at the terminalV_(DD1) of the circuit 13 of FIG. 14. In the circuit of FIG. 15, itshould be noted that the transistors 196, 197, . . . are supplied with aclock signal φ and a logic inversion/φ.

Next, a third embodiment of the present invention will be described withreference to FIG. 16, wherein FIG. 16 shows a device that has a column,redundancy. In FIG. 16, those parts described previously are designatedby the same reference numerals and the description thereof will beomitted.

In the present embodiment, the memory cell array 11 is divided into anumber of memory cell blocks 11₁ . . . 11_(m), wherein each memory cellblock has a corresponding erase power supply 22₁, . . . 22_(m) forsupplying the source drive voltage of the flash memory cells such thatthe flash-erasing of information occurs in each memory cell blockindependently. Further, there is provided a redundant memory cell column11_(CR) corresponding to the column redundant memory cell array shown inFIG. 5, and the redundant memory cell column 11_(CR) receives the sourcedrive voltage from an independent erase power supply 22_(CR). Similar tothe device of FIG. 5, the redundant memory cell block 11_(CR) isselected in response to the output from the defect detection circuit 25.In the illustration of FIG. 16, the redundant decoder 24 is included inthe column decoder 15. In addition, there may be provided a utilitymemory cell column 11_(UT) for testing the device, wherein the memorycell column 11_(UT) is selected in response to a test signal that issupplied to the column decoder 18 when testing the flash memory device.In the memory cell column 11_(UT), too, a power supply 22_(UT) isprovided. In the device of FIG. 16, it should be noted that each of thememory cell blocks 11₁ -11_(m) includes only the real memory cells. Inother words, the memory cell blocks 11₁ -11_(m) does not include theredundant memory cell column or utility memory cell column.

FIG. 17 is a circuit diagram showing a part of the device of FIG. 16 indetail. In FIG. 17, those parts described previously are designated bythe same reference numerals and the description will be omitted.

Referring to FIG. 17, it will be noted that n bit lines such as the bitlines BL₁ -BL_(n) are grouped to form a memory cell block such as 11₁, .. . 11_(m) wherein no redundant memory cell column is included in eachmemory cell block. There, each memory cell block is activated by acorresponding power supply unit such as 22₁, . . . 22_(m) via sourcesupply lines SL₁ -SL_(m) and the flash-erasing or simultaneous erasingof information is achieved within each memory cell blocks. According tothe construction of FIG. 17, the area that is occupied by the redundantmemory cell column can be reduced as compared with the conventionalredundant construction wherein the redundant memory cell column isprovided in each of the memory cell blocks. As the redundant memory cellcolumn is activated explicitly by the power supply unit 22_(CR), theerasing of the redundant memory cell block 11_(CR) does not cause theproblem of the excessive erasing in the real memory cell array 11. Ofcourse, there may be a plurality of memory cell columns included in theredundant memory cell column 11_(CR).

FIG. 18 shows a modification of the device of FIG. 16 wherein the memorycell array 11 is formed of a single row and column formation of thememory cell transistors M₁,1 -M_(m),m, wherein the device furtherincludes the utility memory cell column 11_(UT). As shown in FIG. 18,the utility memory cell column 11_(UT) includes memory cells M₁,U, M₂,U,. . . wherein each of the memory cells M₁,U, . . . have the sourceregion connected commonly by a source supply line SL_(U) to the powersupply unit 22_(UT) that is provided independently from the power supplyunit 22 that supplies the source voltage to the memory cell transistorsin the memory cell array 11. There, the power unit 22_(UT) changes thesupply voltage in response to the operational mode particularly in theerasing mode as already described with reference to the basic operationof the flash memory device such that the supply voltage is changedindependently to the supply voltage that is supplied to the source ofthe memory cell transistors M₁,1 -M_(m),m from the power supply unit 22.

The utility memory cell column 11_(UT) of FIG. 18 may be used forexample for the testing purpose for guaranteeing the number of times thedevice is capable of rewriting as already described, wherein the deviceof the present embodiment is advantageous in the point that the erasingoperation of the utility memory cell column 11_(UT) does not affect atall the state of the real memory cell array 11 because of the use of theseparate, independent power supply unit 22_(UT) for effecting aconnection of the bit line BL_(U) to the sense amplifier 20. Inoperation, a test control signal is supplied to the gate of the columngate transistor (Tsw)_(UT). It should be noted that such a testing ofthe device has been impossible in the conventional device that uses onlyone power supply unit, as such a test for rewriting data in the utilitymemory cell column inevitably causes an excessive erasing in the realmemory cell array unless rewriting of data is achieved simultaneously tothe utility memory cell column. Of course, such a simultaneous rewiringof the data in the real memory cell array inevitably invites shortenedlifetime of the device before the device is actually shipped to theuser. The present embodiment can solve this problem successfully withoutcomplicating the design of the device.

Next, a fourth embodiment of the present invention will be describedwith reference to FIG. 19, wherein those parts corresponding to theparts described previously are designated by the same reference numeralsand the description will be omitted.

Referring to FIG. 19, the memory cell array 11 is divided into aplurality of memory cell blocks 11₁ -11_(m), wherein the device furtherincludes a redundant memory cell column 11_(CR) and a utility memorycell column 11_(UT). Each memory cell block includes a plurality of bitlines connected commonly to a source of a MOS transistor that forms theswitch S₁ to be described below. There, the memory cell blocks 11₁-11_(m) as well as the memory cell columns 11_(CR) and 11_(UT) areconnected to the sense amplifier 20 and the write amplifier 19 via acommon output line DL wherein there are provided switches SW₁ -SW_(CR)for controlling the connection between the bit lines in the memory cellblocks 11₁ -11_(m) and the output line DL as well as the connectionbetween the bit lines included in the memory cell columns 11_(UT) and11_(CR) and the output line DL. There, the switches SW₁ -SW_(CR) aresupplied with a control signal S₁ -S_(CR) and are activated in responsethereto. Further, each of the memory cell blocks and memory cell columns11₁ -11_(CR) has a corresponding power supply unit 22₁ -22_(CR) forsupplying the source voltage to the memory cell transistors includedtherein.

In the present embodiment, it should be noted that the power supplyunits 22₁ -22_(CR) are designed to have respective output power orsupply current that is optimized based upon the number of the memorycells included in the corresponding memory cell block or memory cellcolumn. Hereinafter the optimization of the current output ability ofthe power supply units will be described in brief.

In the flash memory devices, the erasing of information is achieved byremoving the electric charges from the floating electrode gate in theform of tunneling current. Thereby, there is a tendency that holes arecreated as a result of formation of the tunneling current and the holesthus created penetrate into the gate insulation film located under thefloating gate. Thereby, the operational characteristics of the memorycell transistor is deteriorated. In fact, such an accumulation of theholes in the gate insulation film is one of the major reasons thatlimits the lifetime of a flash memory device.

FIG. 20 shows the change of the source current with the source voltageapplied to the memory cell transistor shown in FIG. 1 during the erasingoperation, wherein FIG. 21 shows the condition that the erasingcharacteristics of FIG. 20 is obtained.

Referring to FIG. 21 first, the memory cell transistor shown in FIG. 1is biased according to the erasing condition as set forth in FIG. 2, anda gate current that flows from the floating gate 3 to the source 6 ismeasured by applying a d.c. voltage to the floating gate. As shown inthe result of FIG. 2, the source current I_(S) increases gradually withincreasing source voltage V_(S) until a critical voltage V_(SC) isreached above which the source current I_(S) increases steeply due tothe avalanche breakdown. The erasing of the information is achievedduring this interval where the source current I_(S) increases graduallywith the source voltage V_(S). Once the critical voltage V_(C) isattained, it will be noted that a gate current starts to flow from thefloating gate electrode 3 to the source region 6 as indicated in FIG.2O. It will be noted that such a gate current indicates, in the usualerasing operation of the flash memory device wherein no external supplyvoltage is connected to the floating gate 3, that positive electriccharges are created. In other words, there occurs the problem of theexcessive erasing. Further, the holes thus created penetrate and inducea degradation in the gate insulation film.

Thus, in order to avoid the degradation of the gate insulation film andto maximize the lifetime of the device, it is essential to optimize thesource voltage and hence the source current that is employed during theerasing operation. Particularly, the optimization of the source currentis essential in each or the memory cell blocks and the memory cellcolumns in the configuration as shown in FIG. 19. In the utility memorycell column 11_(UT) in particular, it is essential to supply the sourcevoltage such that the same source current flows in each memory celltransistor therein during the erasing operation as in the memory celltransistors in the memory cell blocks 11₁ -11_(m). Otherwise, the resultof the rewriting test conducted on the utility memory cell columnbecomes useless.

FIGS. 22(A)-22(D) show the construction of the power supply units 22₁-22_(UT), wherein the circuits shown therein have generally a similarconnection. For example, the power supply unit 22₁ has an input terminal22a to which an erase control signal ERS3 is supplied. The erase controlsignal ERS3 thus supplied is further transferred to the gate of ap-channel MOS transistor 22d and an n-channel MOS transistor 22cconnected in series between a supply voltage source V_(pp) and theground via a transfer gate transistor 22b that is urged to the turned-onstate by a supply voltage V_(cc) supplied to the gate thereof. There,the output obtained at the intermediate node between the transistors 22cand 22d is supplied on the one hand to the gate of a p-channel MOStransistor 22c that is connected between the supply voltage source and anode 22g connected to the gate of the transistors 22c and 22d, and onthe other hand to the gate of the transistors 22h and 22j that areconnected in series between the power supply V_(pp) and the ground witha transistor (22i)₁ interposed therebetween. There, the transistor(22i)₁ has a gate and a source connected with each other to form aconstant current circuit, and the output of the circuit 22₁ is obtainedat the node where the transistor (22i)₁ is connected to the transistor22j. As will be noted later, the output transistor (22i)₁ has a gatewidth W₁ that is adjusted to supply an optimized output current to thesource line SL₁ that is connected to the source of the memory celltransistors.

In the normal operation for reading or writing data, the node 22g takesa low level state in response to the low level state of the erasecontrol signal ERS₁ that indicates the non-erasing mode, and thep-channel transistor 22d is turned on while the n-channel transistor 33cis turned off. Thereby, the node 22f is urged to the level V_(pp) andthe p-channel transistors 22e and 22h are turned off while the n-channelMOS transistor 41 is turned on. As a result, the source line SL₁ is setto zero volt.

When the erase signal ERS₁ is set to the high level state incorrespondence to the erasing operation, on the other hand, the level ofthe node 22g is urged to the Vcc level and the transistor 22c is turnedon. As a result, the level of the node 22f approaches to zero and thep-channel transistors 22e and 22h are turned on while the n-channeltransistor 22j is turned off.

In response to the turning-on of the p-channel transistor 22e, the levelof the node 22g rises to the level V_(pp), and the p-channel transistor22d is turned off. As a result, the level of the node 22f is urged tozero volt and the supply voltage V_(pp) is supplied to the source lineSL₁ via the p-channel transistor 22h and the n-channel transistor(22_(i))₁. Thereby, the erasing of information is achieved. A similaroperation is achieved in the other circuits shown in FIGS. 20(B)-20(D).As the operation for these circuits is obvious, further description forthe supply circuits of FIGS. 20(B)-20(D) will be omitted.

In the circuit of FIG. 20(A), it will be noted that the transistor(22i)₁ acting as a constant current source has a gate width W₁ that isset to provide a source current that is sufficient to erase theinformation from the memory cell transistors included in the memory cellblock 11₁. In other words, the gate width W₁ is set in accordance withthe number of the memory cell transistors that are included in thememory cell block. More specifically, the gate width W₁ is set such thatthe power supply circuit 22, has a capability of supplying the currentI_(SC) corresponding to the critical source voltage V_(SC) to each ofthe memory cell transistors included in the memory cell block 11₁. SeeFIG. 20. Thereby, an optimum source current is supplied to the memorycell transistors in the memory cell block 11₁ when erasing information.Similarly, the output transistors (22i)_(m), (22i)_(UT) and (22i)_(CR)have respective optimum gate widths W_(m), W_(UT) and W_(CR). Thereby,there holds a relationship

    W.sub.1 /N.sub.1 =W.sub.m /N.sub.m =W.sub.UT /N.sub.UT =W.sub.CR /N.sub.CR,

where N₁, N_(m), N_(UT) and N_(CR) respectively represent the number ofthe memory cell transistors that are included in the memory cell blocks11₁ and 11_(m) and in the memory cell columns 11_(UT) and 11_(CR). Incorrespondence to the foregoing relationship, the current supplycapabilities P₁, P_(m), P_(UT) and P_(CR) of the circuits 22₁, 22_(m),22_(UT) and 22_(CR) are related to each other according to the followingrelationship

    P.sub.1 /N.sub.1 =P.sub.2 /N.sub.2 =P.sub.3 /N.sub.3 =P.sub.4 /N.sub.4.

Next, a fifth embodiment of the present invention will be described withreference to FIG. 23 that shows the principle of the present embodiment.

Referring to FIG. 23, the present embodiment employs the redundantmemory cell block 11_(CR) and the utility memory cell block 11_(UT),wherein each of the blocks 11_(CR) and 11_(UT) include a plurality ofmemory cells arranged to form a matrix. In other words, the memory cellblocks 11_(CR) and 11_(UT) include a number of memory cell columns.There, it will be noted that the column decoder 15 is provided commonlyto the memory cell blocks 11_(CR) and 11_(UT) for selecting a columnselection line such as BL₁, wherein the selection of the columnselection line BL₁ is achieved simultaneously in the memory cell block11_(CR) and 11_(UT). Thereby, the information read out from the selectedmemory cell column is supplied to the sense amplifier 20 either via aswitch circuit SW₁ or SW₂, wherein the switch circuit SW₁ is closed whenthe redundant memory cell block 11_(CR) is activated. On the other hand,when the utility memory cell block 11_(UT) is selected, the switchcircuit SW₂ is closed. Thereby, the output of the memory cell block11_(UT) and the memory cell block 11_(CR) is supplied to the senseamplifier 20 selectively.

FIG. 24 shows a more detailed representation of the circuit of FIG. 23.

Referring to FIG. 24, it will be noted that the redundant memory cellarray 11_(CR) includes memory cell transistors M_(CR) (1,1), . . .arranged in rows and columns while the utility memory cell array 11_(UT)includes memory cell transistors M_(UT) (1,1), . . . There, theredundant memory cell transistors M_(CR) (1,1), . . . are connected tocolumn lines (CL₁)_(CR), . . . while the utility memory cell transistorsM_(UT) (1,1), . . . are connected to column lines (CL₁)_(UT). The columnlines (CL₁)_(CR), . . . are selected by column gate transistors(Tsw1)_(CR), . . . in response to the output supplied by a final stagecircuit or driver circuit 15a of the column decoder 15 to the columnselection lines BL₁, . . . , while the column lines (CL₁)_(UT), . . .are selected by column gate transistors (Tsw1)_(UT), . . . in responseto the output of the same driver circuit 15a outputted on the columnselection lines BL₁, . . . , There, it will be noted that the columnselection lines BL₁, . . . are connected commonly to the correspondingcolumn gate transistors (Tsw1)_(CR), . . . for the redundant memory cellarray 11_(CR) and to the column gate transistors (Tsw1)_(UT), . . . forthe utility memory array 11_(UT). In each of the memory cell arrays11_(CR) and 11_(UT), the drains of the column gate transistors areconnected to a common drain line (DL)_(CR) and a common drain line(DL)_(UT), wherein the drain line (DL)_(CR) is connected to the senseamplifier 20 via a transfer gate transistor Tr₁ that is supplied with acontrol signal RED for activating the redundant memory cell array andacts as the switch SW₁, while the drain line (DL)_(UT) is connected tothe sense amplifier 20 via a transfer gate transistor Tr₂ that issupplied with a control signal TEST for activating the testing in theutility memory cell array and acts as the switch SW₂. As usual in thecolumn redundant memory circuits, the control signal RED is produced inresponse to the column address data upon the selection of a defectivebit line. See for example the circuit of FIG. 5, wherein the signal REDis produced by the redundant decoder 24. On the other hand, the controlsignal TEST is produced externally upon the running of the testprocedure.

In the present construction, the memory cell transistors are selectedsimultaneously in the redundant memory cell array 11_(CR) and theutility memory cell array 11_(UT), wherein the information signals thusread out from the selected memory cell transistors are selected furtherby the transistors Tr₁ and Tr₂ in response to the control signals REDand TEST supplied thereto.

In FIG. 24, it will be noted that the control signals RED and TEST aresupplied also to the column driver 15a via an OR gate 15b for activatingthe same. FIG. 25 shows the construction of the OR gate 15b as a part ofthe driver circuit 15a.

Referring to FIG. 25, it will be noted that the driver circuit 15aincludes a number of circuit blocks 40₀ -40₃ provided in correspondenceto the column selection lines BL₁ -BL₄, wherein each of the circuitsblocks 40₀ -40₃ have the same construction and only the circuit block40₀ will be described.

The circuit block 40₀ includes a depletion mode n-channel MOS transistor41₀ and an enhancement mode n-channel MOS transistor 43₀ connected inseries, wherein the drain of the transistor 41₀ is connected to thesupply voltage V_(cc), while the source and drain of the transistor 41₀are connected to each other at a node 47₀. Further, a selection signal/(A_(n) +A_(m)) produced as a result of the decoding in the decodingpart of the column decoder 15 is supplied to the gate of the transistor43₀, wherein the transistor 43₀ has a source that is connected to theground either via a transfer gate transistor 45 or the transfer gatetransistor 46. There, it will be noted that the transistors 45 and 46form a part of the OR gate circuit 15b. In response to the controlsignal RED or TEST, the transistor 45 or 46 is turned on and thetransistor 43₀ supplies an output signal to the foregoing node 47₀ inresponse to the selection signal /(A_(n) +A_(n+1)) supplied to the gateof the transistor 43₀, provided of course that the transistor 45 or 46is turned on.

The output signal at the node 47₀ is supplied further to the gate of thep-channel transistor 42₀ and simultaneously to the gate of the n-channeltransistor 44₀ connected in series thereto, wherein the transistors 42₀and 44₀ form an output stage circuit of the circuit block 40₀ andconnected between the supply voltage V_(cc) and the ground. There, theoutput stage circuit outputs the column selection signal on the columnselection line BL₁ that is connected to the intermediate node betweenthe transistors 42₀ and 44₀.

It will be noted that each of the circuit blocks 40₁ -40₃ conducts thesame operation in response to various combinations of the input signalssuch as /(/A_(n) +A_(n+1)), /(A_(n) +/A_(n+1)) and /(/A_(n) +/A_(n+1)),wherein the drain of the transistors 43₀ -43₃ are connected commonly tothe transistors 45 and 46.

According to the present embodiment shown in FIG. 24, the area of thesemiconductor chip that has been occupied by the column decoder can bereduced substantially, as the redundant memory cell array 11_(CR) andthe utility memory cell array 11_(UT) use the same column decoder.Further, it should be noted that this feature is not only valid in theflash memory devices but also to other conventional semiconductormemories such as dynamic random access memories.

Next, a sixth embodiment of the present invention will be described withreference to FIG. 26 that corresponds to a modification of theconstruction of the circuit of FIG. 5.

Referring to FIG. 26, the circuit of the present embodiment is intendedfor testing the erasing operation of the flash memory device andincludes a write control circuit 25a.

In the conventional column-redundant flash memory device of FIG. 5, itshould be noted that the conventional circuit of FIG. 5 selects aredundant memory cell column when a defective memory cell column isaddressed. Thereby, the writing of the defective memory cell column isnot made and the memory cells included in the defective memory cellcolumn become excessively erased state upon the flash-erasing process.In the circuit of FIG. 5, this excessive erasing of the defective columndoes not cause any problem, as the selection of such a defective columnis prohibited by the column gate transistor such as Tsw₃.

When testing a newly fabricated device for identifying the defectivememory cells with respect to the erasing performance, on the other hand,the foregoing construction has a drawback in that the writing orinformation into the redundant memory cell column is not possible in theabsence of information about the defective memory cells. Morespecifically, the writing of data "0" into the memory cells beforeerasing for avoiding the excessive erasing cannot be conducted for theredundant memory cells. Thereby, the erasing test conducted upon thereal memory cells connected to real bit lines BL₁ -BL_(n) inevitablycauses an excessive erasing state in the redundant memory cellsconnected to the redundant bit lines (BL₁)_(CR) -(BL₂₁)_(CR).

In order to avoid the foregoing problem, the circuit of FIG. 26 employsa write control circuit 25a that activates the redundant column decoder24 via an OR gate 25b in response to a redundant selections signal REDACTIV. Simultaneously, the circuit 25a deactivates the column decoder 15also via the OR gate as well as via an inverter 25b. Thus, when thesignal RED ACTIV is high, the decoder 24 is activated and the writing ofdata is possible into the redundant memory cell columns, while when thesignal RED ACTIV is set low, the decoder 24 is deactivated and thewriting of data into the redundant memory cell columns is prohibited. Inaddition, it will be noted that the circuit of FIG. 26 achieves theusual column redundancy controlled by the defect detection circuit 25via the OR gate 25b.

Thus, when conducting the foregoing erasing test, a control signal W issupplied to the decoders 13, 15 and 24 such that the signal W has a highlevel state corresponding to the voltage V_(pp) for setting the memorycell transistors in the real memory cell array as well as in theredundant memory cell array to the state ready for writing information.Further, the control signal RED ACTIV is set to the low level state andthe writing of the data "0" is achieved into the memory cell transistorsin the real memory cell array while prohibiting the wiring of the data"0" into the redundant memory cell arrays. Next, the control signal REDACTIV is set to the high level state and the writing of the data "0" isconducted into the redundant memory cell transistors while prohibitingthe writing into the real memory cell transistors. Further, after thereal memory cell transistors and the redundant memory cell transistorsare all written with the data "0," the erase power supply 22 isactivated in response to the erase control signal E, and theflash-erasing of information is achieved for the entirety of the memorycells including the real memory cells and the redundant memory cells.

FIG. 27 shows an example of the construction of the circuit 25a shown inFIG. 26, wherein the circuit includes p-channel MOS transistors 251 and252 as well as an n-channel MOS transistor 253 connected in series witheach other between the two supply voltages V_(cc) and V_(ss), whereinthe transistors 252 and 253 have respective gates connected each otherto the supply voltage V_(cc). Thereby, the control signal RED ACTIV issupplied to a terminal P_(in) connected to the drain of the transistor251 and takes a voltage level V_(HH) that exceeds the supply voltageV_(CC) when the signal RED ACTIV is in the high level state. Thus, whenthe signal RED ACTIVE is high, the p-channel transistor 252 is turned onand the n-channel transistor 253 is turned on and a voltage at the nodebetween the transistors 252 and 253 is supplied to an inverter circuitthat includes a series connection of the p-channel transistor 254 and ann-channel transistor 255. Further, the output of the inverter issupplied to a next inverter that includes a series connection of ap-channel transistor 256 and an n-channel transistor 257, and the outputcontrol signal to be supplied to the OR gate 25b is obtained at theintermediate node between the transistors 256 and 257.

FIG. 28 shows a flowchart for separating defective produces from goodproducts based upon the foregoing erasing test operation.

Referring to FIG. 28, the wring of data "0" is conducted into the realmemory cells in a first step 1 by activating the column address buffercircuit 14 and the column decoder 15. Next, the writing of the data "0"is conducted into the redundant memory cells in a step 2 by activatingthe redundant decoder 24 via the write control circuit 25a.

After the memory cells in the real and redundant memory cell arrays areall written with the data "0", an erasing process is achieved in a step3 by activating the erase power supply unit 22. Thereby, the flasherasing of information is achieved for the entirety of the memory cellsin the real and redundant memory cell arrays.

Next, the result of the flash-erasing process of the step 3 is verifiedby reading the content of information of the memory cells in the realand redundant memory cell arrays. When it is discriminated in a step 5that all the memory cells are subjected to the proper erasing process,the device is judged in a step 6 as being a good product.

On the other hand, when an erroneous erasing was found, the addresses ofthe defective memory cells that show the erroneous erasing are writteninto a memory that is provided in the defect detection circuit 25 in astep 7. Further, a verification process is conducted in a step a forverifying the column redundant operation of the device based upon theaddress of the defective memory cells.

When it is confirmed in a step 9 that the column redundant operation issatisfactory, the step 6 is conducted and the device is identified asbeing a good product. on the other hand, when the result ofdiscrimination in the step 9 is unsatisfactory, a step 10 is conductedto achieve the erasing procedure again. Further, the operation of thedevice is verified in a step 11 by reading the content of the memorycells. Further, the result of the verification process of the step 11 ischecked in a discrimination step 12 to discriminate whether the deviceachieves the satisfactory operation or not. If the result in the step 12is YES, the step 6 is conducted and the device is identified to be agood product. On the other hand, when the result in the step 12 is NO, astep 13 is conducted wherein the device is identified as being adefective product.

Next, a seventh embodiment of the present invention will be describedwith reference to FIG. 29 that shows the principle of the embodiment.

Referring to FIG. 29, the present embodiment is based upon a conceptsimilar to the concept of the embodiment of FIG. 23 and uses a commoncolumn decoder for the real memory cell array 11 and redundant memorycell arrays 11_(CR1) -11_(CRn). Thereby, the construction for realizingthe column redundancy is substantially simplified, As the circuit ofFIG. 29 is designed for the column redundancy instead of the embodimentof FIG. 23 that is designed for the testing of the redundant memory cellcolumn and the utility memory cell column, there exists a differencebetween the circuit of FIG. 29 and the circuit of FIG. 23 as will bedescribed below.

Referring to FIG. 29, it will be noted that the redundant decoder 24used in the construction of FIG. 5 is no longer used in the presentembodiment. Thereby, the column decoder 15 is used commonly for the realmemory cell array 11 and the redundant memory cell arrays 11_(CR1)-11_(CRn), and the selection of a bit line such as B₁ in the memory cellarray 11 causes a simultaneous selection of the bit lines in theredundant memory cell arrays. Thus, in order to transfer the informationstored in a selected memory cell to the sense amplifier 20, there isprovided a switch circuit 16A such the switch circuit 16A is activatedin response to a control signal S_(comp) outputted from the defectdetection circuit 25.

FIG. 30 shows the essential part of the circuit of FIG. 29, wherein itwill be noted that the column decoder 15 includes a number of decodingcircuits DC₀, DC₁, . . . corresponding respectively to the column gatetransistors T_(a), T_(b), T_(c), T_(d), . . . , wherein the decodingcircuit DC₀ selects the bit line B₀ while the decoding circuit DC₁selects the bit line B₁. There, the transistors T_(a), T_(b), . . .correspond to the previously described column gate transistors Tsw₁, . .. and form the 16, 16_(CR1), 16_(CR2), . . . Further, the defectdetection circuit 25 raises the level of one of the control signalsS_(B0), S_(B1), . . . that are supplied therefrom to the decodingcircuits DC₀, DC₁, . . . , selectively based upon the result ofcomparison of the supplied column address data with the address data ofthe defective memory cell columns. Thus, when an addressing of adefective memory occurs, a decoder such as DC₁ is selectively activated.In this case, the bit lines B₁ and B_(1s) are selected simultaneously.

Simultaneously to the foregoing control of the column decoder 15, thedefect detection circuit 25 supplies the control signal S_(comp) to theswitch circuit 16A. It will be noted that the switch circuit 16Aincludes transfer gate transistors T_(e) and T_(f) wherein thetransistors T_(e) and T_(f) are turned on and turned off complementaryin response to the control signal S_(comp). It should be noted that thetransistor T_(e) is provided in correspondence to the real memory cellarray 11 and the bit lines B₀, B₁, . . . in the real memory cell array11 are connected commonly to the transistor T_(e) via respective columngate transistors T_(a), T_(b), . . . Similarly, the redundant bit linesB_(0S), B_(1S) of the redundant memory cell arrays are connectedcommonly to the transistor T_(f) that corresponds to the redundantmemory cell array.

FIG. 31 shows the construction of the decoding circuit such as DC₀wherein it will be noted that p-channel transistors T_(j), T_(g), T_(n),. . . , T_(i) are connected in series between the supply voltage V_(CC)and the supply voltage V_(SS), wherein the transistors T_(g), T_(n), . .. T_(i) are supplied with a logic combination of the column addresssignals and cause a transition to the turned-on state in correspondenceto a particular logic combination thereof, while the transistor T_(j)acts as a constant current source and supplies a drive current to a nodeN where the transistor T_(j) and the transistor T_(g) are connected witheach other. There, the output obtained at the node N as a result of theforegoing decoding action of the transistors T_(g) -T_(i) is supplied toan output inverter that includes a series connection of a p-channel MOStransistor T_(OUT) and an n-channel MOS transistor T_(OUT), as usual,wherein there is provided an additional transistor T_(k) such that thetransistor T_(k) is connected between the node N and the ground G.There, the transistor T_(k) is supplied with the foregoing signal S_(B0)or S_(B1) from the defect detection circuit 25 at the gate thereof andurges the level of the node N at the low level state in response to thehigh level state of the control signal S_(B0) or S_(B1), irrespective ofthe logic combination of the column address signal. Thereby, the outputlevel of the output inverter is forced to the high level state. In otherwords, the control signals S_(B0) and S_(B1) from the defect detectioncircuit 25 overrides the result of the decoding in the column decoder,and the selection of the redundant bit line occurs irrespective of thedecoding operation in the column decoder 15, as long as the columnaddress data specifies a defective memory cell column.

FIG. 32 shows a modification of the embodiment of FIG. 29, wherein thereal memory cell array 11 is divided into a plurality of memory cellblocks 11₁ and 11₂. In correspondence to this, redundant memory cellblocks 11_(CR1) and 11_(CR2) are provided. There, the bit lines in thememory cell block 11₁ are selected by a column gate switch circuit 16₁,the bit lines in the memory cell block 11₂ are selected by a column gateswitch circuit 16₂, while the bit lines in the memory cell block11_(CR1) are selected by a column gate switch circuit 16_(CR1), and thebit lines in the memory cell block 11_(CR2) are selected by a columngate switch circuit 16_(CR2).

In this construction, a redundant decoder 24A similar to the redundantdecoder 24 of FIG. 5 is used for activating the column gate switches16₁, 16₂, 16_(CR1) and 16_(CR2), wherein the column gate switches 16₁and 16₂ are controlled by the column decoder as usual. On the otherhand, the column gate switches 16_(CR1) and 16_(CR2) are activatedcommonly by a control signal SR_(SEL) produced by a redundant decoder24A. There, the redundant decoder 24A is supplied with the columnaddress data from the column buffer 14 simultaneously to the columndecoder 15 and controls the decoder 15 by supplying a control signalS_(INH) such that the operation of the decoder 15 is prohibited whenthere exist a bit line in the real memory cell 11₁ or 11₂ and also a bitline in the redundant memory cell 11_(CR1) and 11_(CR2) incorrespondence to a given column address. For example, when a defectivecolumn line is selected, the decoder 24A prohibits the operation of thedecoder 15 and selects redundant bit lines in the memory cell arrays11_(CR1) and 11_(CR2). Thereby, the reading of information is achievedeither from the real memory cell array or from the redundant memory cellarray. For example, the reading is achieved either from the memory cellarray 11₁ or 11_(CR1), and the information thus read out is supplied tothe transistor T_(e) that forms a part of the switch circuit 16A.Alternatively, the reading of information is achieved either from thememory cell array 11₂ or from the redundant memory cell array 11_(CR2)and the information thus read out is supplied to the transistor T_(f)included in the switch circuit 16A.

There, the decoder 24A further produces control signals SR₁ and SR₂ andactivates either the transistor T_(e) or the transistor T_(f) inresponse to the logic combinations of the signals SR₁ and SR₂ via acontrol circuit 24B. As a result, reading of information is achievedselectively from either of the memory cell arrays 11₁, 11₂, 11_(CR1) and11_(CR2).

FIG. 33 shows the construction of the circuit 24B.

Referring to FIG. 33, the circuit 24B includes a NOR gate 241 and a NANDgate 242 connected in series wherein the supply voltage V_(SS) issupplied to one input terminal of the NOR gate 241 while the mostsignificant is bit A_(MSB) of the column address data is supplied to theother input terminal. The output of the NAND gate 242 is supplied to aNOR gate 243, wherein the NOR gate 243 is supplied with the controlsignal SR₁ at the other input terminal. Further, the output of the NORgate 243 is supplied to a first input terminal of a NOR gate 244 that issupplied also with the control signal SR₂ at the other input terminal.Further, the output of the NOR gate 244 is supplied to the transistorT_(e) as a control signal SEL and further to the transistor T_(f) via aninverter 245 as a control signal SELx. There, it will be noted that theoutput signals SEL and SELx are produced as a result of the logiccombination of the signals A_(MSB), SR₁ and SR₂. There, the signal SELtakes the same logic level as the signal A_(MSB) when the signals SR₁and SR₂ both have the low level state in correspondence to thenon-redundant operation. On the other hand, the logic level of theoutput signal SEL is urged to the high level state when the signal SR₁has the high level state. Further, the signal SEL is urged to the lowlevel state when the signal SR₂ has the high level state.

The present modification as described with reference to FIGS. 32 and 33is also effective for simplifying the construction of the memory deviceby controlling the redundant memory cell arrays 11_(CR1) and 11_(CR2) bythe same control signal SR_(SEL) produced by the same decoder circuit24A.

Next, an eighth embodiment of the present invention will be describedwith reference to FIG. 34.

In the flash memory devices, attempts are made to reduce the voltagethat is applied to the source region of the memory cell transistor. Inthe conventional erasing process explained with reference to FIG. 2, itwill be noted that a very high voltage (V_(H)) such as 12 volts has tobe applied to the drain region of the memory cell transistor. On theother hand, the application of such a high voltage to the n⁺ -typesource region tends to invite a breakdown at the p-n junction betweenthe source region and the substrate. Further, the application of such avery large voltage to the source region tends to induce a depletionregion in the substrate immediately under the gate insulation film, andthe large electric field associated with such a depletion region maycause to flow a tunneling current from the valence band to theconduction band. Thereby, an unnecessarily large current flows in theform of the source current, when erasing information. In addition, sucha large electric field tends to cause an injection of holes into thegate insulation film, and the injection of holes deteriorates theoperational characteristics and the lifetime of the flash memory device.

The erasing process explained already with reference to FIGS. 11 and 12for the second embodiment avoids this problem by applying a largenegative voltage to the control gate electrode such that the necessityof applying a high voltage to the source region is eliminated.

In the present embodiment, the same goal as the second embodiment isachieved by applying simultaneously a positive voltage to the substrateand a negative voltage to the control gate electrode, when erasinginformation, with the same magnitude such that no extraordinary largevoltage appears between the active parts of the memory cell transistorsas well as the peripheral transistors forming the peripheral circuit.

FIG. 34 shows the foregoing principle of the present embodiment as wellas the device structure of the present embodiment that is tailored forimplementing the foregoing principle.

Referring to FIG. 34, the flash memory is constructed on a p-typesubstrate 110 that is defined with a memory cell region wherein memorytransistors A and B are formed and a peripheral region wherein aperipheral transistor is formed. There, the memory cell region includesn⁺ -type diffusion regions 126a and 128a respectively serving for thesource and drain of the memory cell transistor A and diffusion regions126b and 128b respectively serving for the source and drain of thememory cell transistor B. Further, the memory cell transistors A and Bare isolated from each other by a field oxide region 116, and a gateinsulation film 118 covers the surface of the device region for thetransistors A and B as usual in MOS transistors.

On the gate insulation film 118, a floating gate electrode 120a isprovided in correspondence to the memory cell transistor A, while afloating gate electrode 120b is provided on the gate insulation film 118in correspondence to the memory cell transistor B. On the floating gateelectrodes 120a and 120b, capacitor insulation films 122a and 122b areformed respectively, and control gate electrodes 124a and 124b areprovided respectively on the capacitor insulation films 122a and 122b.

Further, it will be noted that a p-type well 114 surrounded by anexternal well 112 is formed in the substrate 110 in correspondence tothe peripheral region, and n⁺ -type diffusion regions 132 and 134 areformed in the well 114 as the source and drain of the peripheraltransistor. As usual, the gate insulation film 118 in formed also on thesurface of the substrate 110 in correspondence to the peripheraltransistor and a gate electrode 130 is provided thereon.

When erasing information from the memory cell transistor A, for example,the present embodiment applies a gate voltage of -10 volts to thecontrol gate electrode 124a while simultaneously applies a substratevoltage of +10 volts. Thereby, a voltage difference of 20 volts isformed between the control gate electrode 124a and the substrate 110,and the electrons accumulated in the floating gate 120a are expelled tothe substrate 110 in the form of the Fowler-Nordheim tunnel current. Inorder to avoid the unwanted erasing of information, a positive voltageof +10 volts is applied simultaneously to the control gate 124b of thememory cell transistor. Thereby, it will be noted that the voltagedifference between the control gate 124b and the substrate 110 becomeszero.

When implementing the foregoing principle for erasing information, itwill noted that the voltage level of the substrate 110 increasespositively also in the peripheral region if the substrate of theperipheral region is not isolated from the rest of the part of thesubstrate in the form of double well structure shown in FIG. 34. There,the positive voltage level applied to the p-type substrate inevitablyinduces a forward biasing at the p-n junction between the substrate andthe diffusion regions 132, 134.

The present embodiment avoids this problem of adversary forward biasingby providing an n-type well 112 in the substrate 110 in correspondenceto the peripheral region and further by forming a p-type well 114 withinthe n-type well 112. Thereby, the diffusion regions 132 and 134 areformed within the p-type well 114.

In operation, a positive voltage set equal to the positive voltageapplied to the substrate 112 is applied to the n-type well 112 such thatthere occurs no substantial biasing between the p-type substrate 110 andthe n-type well. Further, the voltage level of the p-type well 114 isheld at zero volt. Thereby, the p-n junction at the interface betweenthe n-type well 112 and the p-type well 114 are reversely biased, andthe depletion region developing along the p-n junction effectivelyisolates the p-type well 114 from the p-type substrate 110. Thus, thestructure of FIG. 34 enables the effective erasing of informationwithout providing excessive electric stress to any part of the device.

In the structure of FIG. 34, the reading and writing of information isachieved similarly to the conventional process shown in FIG. 2. Further,when erasing information, one may apply the same positive voltage as thesubstrate voltage to the source and drain regions of the memory celltransistor.

FIG. 35 is a modification of the device of FIG. 34, wherein the memorycell region is isolated from the peripheral region by a double wellisolation structure. There, it will be noted that the double wellincludes an outer n-type well 136 and an inner p-type well 138, and thediffusion regions of the memory cell transistors A and B are all formedwithin the inner p-type well 138. As the rest of the feature isidentical with the embodiment of FIG. 34, further description of thestructure of FIG. 35 will be omitted.

Next, the fabrication process of the deice of FIG. 34 will be describedwith reference to FIGS. 36(A)-36(F).

Referring to FIG. 36(A), the n-type well 112 is formed in the substrate110 in correspondence to the peripheral region by means of an ionimplantation process of an n-type dopant such as As or P followed by athermal annealing process. Next, the p-type well 114 is formed in then-type well 112 by means of an ion implantation process of a p-typedopant such as B.

Next, the surface of the substrate 110 is protected by an oxidationresistant mask (not shown) such as silicon nitride in correspondence tothe region where the active part of the device is formed, and thesubstrate 110 thus masked is subjected to a thermal oxidation processconducted in a wet O₂ environment. Thereby, the field oxide region 16 isformed. Next, the mask is removed and the gate oxide film 118 is formedby a thermal oxidation process conducted in a dry O₂ environment.Thereby, the structure shown in FIG. 36(B) is formed.

Next, a first polysilicon layer is deposited and patterned subsequentlyto form the floating gate electrodes 120a and 120b respectively incorrespondence to the memory cell transistors A and B as shown in FIG.36(C). Further, the structure of FIG. 36(C) is subjected to a thermaloxidation process to form the capacitor insulation films 122a and 122bon the floating electrodes 120a and 120b, respectively, and a secondpolysilicon layer 124 is deposited further thereon. Thereby, thestructure shown in FIG. 36(D) is obtained.

Next, in the step of FIG. 36(E), the polysilicon layer 124 is patternedto form the control gate electrodes 124a and 124b as well as the gateelectrode 130. Further, in the step of FIG. 36(F), ion implantation ofthe n-type dopant such as As or P is conducted to form the diffusionregions 126a, 126b, 128a and 128b while using the gate structure as aself-alignment mask in each of the memory cell region and the peripheralregion.

In any of the foregoing embodiments, one may employ a laminatedstructure of silicon nitride sandwiched by a pair of silicon oxide filmsfor the capacitor insulation film 4 shown in the memory cell transistorof FIG. 1. By employing such a laminated structure, it is possible toreduce the thickness of the capacitor insulation film withoutsacrificing the reliability and anti-leak characteristics of the device.Thereby, an efficient capacitor coupling is achieved between the controlelectrode and the floating gate electrode. It should be noted that thesilicon oxide film formed on the-polysilicon floating gate shown in FIG.1 tends to form pinholes when the thickness is reduced due to the effectof the grain boundaries in the gate electrode. By covering the thinsilicon oxide film by a silicon nitride film and form a thin siliconoxide film further on the silicon nitride film, one can successfullyeliminate the leak from the floating gate while reducing the thicknessof the capacitor insulation film.

FIG. 37 shows the structure of such a memory cell transistor that hasthe laminated structure for the capacitor insulation film 4. In FIG. 37,those parts correspond to the parts shown in FIG. 1 are represented bythe same reference numerals. There, it will be noted that the capacitorinsulation film 4 includes a silicon oxide film 4a formed directly onthe floating gate electrode 3, a silicon nitride film 4b deposited onthe silicon oxide film 4a, and a silicon oxide film 4c formed on thesilicon nitride film 4b.

In the embodiments described heretofore, it should be noted that thepower supply unit 22 as well as the power supply units 22₁, . . . may bea switching circuit for switching the source voltage between a highlevel voltage such as +12 volts and a low level voltage such as +5volts, instead of the voltage generator circuit that generates the highvoltage output and the low voltage output. In such a construction, twovoltage sources, the first for providing the high voltage and the secondfor providing the low voltage are used, and the power supply unit merelyswitches the output voltage in response to a control signal.

Further, the present invention is not limited to the embodimentsdescribed heretofore, but various variations and modifications may bemade without departing from the scope of the invention.

What is claimed is:
 1. A flash-erasable semiconductor memory devicecomprising:a memory cell array including a plurality of memory celltransistors, each of said memory cell transistors comprisingan insulatedfloating gate provided on a semiconductor substrate with a separationtherefrom for storing information in the form of electric charges; agate insulation film provided on an upper major surface of saidsemiconductor substrate for separating said floating gate from saidsemiconductor substrate; a channel region defined in said semiconductorsubstrate in correspondence to said floating gate; a source region and adrain region defined in said semiconductor substrate at both sides ofsaid floating gate, said source region injecting carriers into saidchannel region such that said carriers are transported along saidchannel region while said drain region collecting carriers that havebeen injected into said channel region at said source region andtransported through said channel region; and a control electrodeprovided on said floating gate with a separation therefrom by acapacitor insulation film for controlling an injection of carriers fromsaid channel region to said floating gate via said gate insulation film;said semiconductor memory device further comprising a column decodersupplied with column address data for selecting a bit line to which thedrain of a selected memory cell transistor is connected, as a result oflogic operation of said column address data; an erase power supply forerasing information from a plurality of memory cell transistors includedin said memory cell array simultaneously, said erase power supplyerasing information by applying an erase voltage between said controlgate electrode and one of said source region, said channel region andsaid drain region such that electric charges are removed from saidfloating gate electrodes of said memory cell transistors in the form oftunneling current flowing through said gate insulation film; whereinsaid memory cell array comprises a main memory cell array and anadditional memory cell array, said main memory cell array and saidadditional memory cell array including respectively at least one bitline to which at least one memory cell transistor is connected; saidcolumn decoder comprising a main decoder for selecting one of the bitlines in said main memory cell array in response to said address datathat is supplied thereto as a result of logic operation of said addressdata and a secondary decoder provided in correspondence to saidadditional memory cell array for selecting a bit line in said additionalmemory cell array in response to an external control signal suppliedthereto; said erase power supply comprising a main erase power unitcorresponding to said main memory cell array and an additional erasepower unit corresponding to said additional memory cell array, saidadditional erase power unit being connected, when erasing information,to said memory cell transistor in said additional memory cell array viaa bit line therein to induce a voltage difference between said floatinggate and a part of said substrate including said source region, drainregion and said channel region of said memory cell transistor, saidadditional erase power unit thereby applying an erase voltage to saidbit line for erasing information from said additional memory cell array;said main memory cell array being formed of a plurality of memory cellblocks each including a plurality of bit lines; said main erase powerunit being formed of a plurality of erase power sub-units incorrespondence to said plurality of memory cell blocks, each of saiderase power sub-units being connected, when erasing information, to saidmemory cell transistor included in said corresponding memory cell blockto induce a voltage difference between said floating gate and a part ofsaid substrate including said source region, drain region and saidchannel region such that the information stored in the memory celltransistors in said corresponding memory cell block is erased.
 2. Aflash-erasable semiconductor memory device, comprising:a memory cellarray including a plurality of memory cell transistors, each of saidmemory cell transistors comprising:an insulated floating gate providedon a semiconductor substrate with a separation therefrom; a gateinsulation film provided on an upper major surface of said semiconductorsubstrate so as to separate said floating gate from said semiconductorsubstrate; a channel region defined in said semiconductor substrate incorrespondence to said floating gate electrode; a source region and adrain region defined in said semiconductor substrate at both sides ofsaid floating gate; a control electrode provided on said floating gatewith a separation therefrom by a capacitor insulation film; and aperipheral circuit, said peripheral circuit includinga word decodersupplied with address data for selecting a word line to which thecontrol electrode of a selected memory cell transistor is connected, asa result of logic operation of said address data; a write controlcircuit for controlling writing of information into said selected memorycell transistor by supplying a write control voltage to said worddecoder in response to a write enable signal such that said writecontrol voltage is supplied to the control gate electrode of saidselected memory cell transistor via said word decoder; an erase boostercircuit for producing an erase voltage, said erase booster circuitsupplying said erase voltage to said word decoder with a magnitudesufficient to expel the electric charges in the floating gate of saidselected memory cell transistor through the gate insulation film bytunneling effect; a power supply circuit for supplying a positivevoltage signal to said source region such that said positive voltagesignal holds the level of said source region of said memory celltransistors at a positive voltage level when erasing information; saidpower supply circuit producing said positive voltage signal with amagnitude substantially equal to or smaller than the voltage applied tothe control gate of said selected memory cell transistor, said erasevoltage thereby being divided into said voltage applied to said controlgate and said positive voltage applied to said source region, whereinsaid substrate includes a first well of a first conductivity typeopposite to a conductivity type of said channel region, and further asecond well of a second conductivity type opposite to the firstconductivity type, such that said second well is formed inside saidfirst well, said second well including therein a transistor forming saidperipheral circuit and wherein said first and second wells are reverselybiased by applying a bias voltage thereto in an erase mode operation ofsaid flash-erasable semiconductor memory device, said erase voltagebeing applied to said first well via a diffusion region of saidtransistor formed in said first well.
 3. A flash-erasable semiconductormemory device as claimed in claim 2, wherein said capacitor insulationfilm has a laminated structure including a silicon nitride layersandwiched by a pair of silicon oxide layers.
 4. A flash-erasablesemiconductor memory device as claimed in claim 2, wherein said powersupply circuit supplies a positive voltage to the source of saidselected memory cell transistor.
 5. A flash-erasable semiconductormemory device as claimed in claim 2, wherein said booster circuitcomprises a plurality of diodes connected in series, each of said diodesbeing supplied with electric charges for accumulating the same toproduce a large negative voltage as said erase voltage, said boostercircuit having an end connected to said substrate and another endconnected to said word decoder such that said large negative voltage isapplied to the control gate of said selected memory cell transistor. 6.A flash-erasable semiconductor memory device, comprising:a memory cellarray including a plurality of memory cell transistors, each of saidmemory cell transistors comprising:an insulated floating gate providedon a semiconductor substrate with a separation therefrom for storinginformation in the form of electric charges; a gate insulation filmprovided on an upper major surface of said semiconductor substrate forseparating said floating gate from said semiconductor substrate; achannel region defined in said semiconductor substrate in correspondenceto said floating gate electrode; a source region and a drain regiondefined in said semiconductor substrate at both sides of said floatinggate, said source region injecting carriers into said channel regionsuch that said carriers are transported along said channel region whilesaid drain region collecting carriers that have been injected into saidchannel region at said source region and transported through saidchannel region; and a control electrode provided on said floating gatewith a separation therefrom by a capacitor insulation film forcontrolling an injection of carriers from said channel region to saidfloating gate via said gate insulation film; and a peripheral circuit,said peripheral circuit includinga word decoder supplied with addressdata for selecting a word line to which the control electrode of aselected memory cell transistor is connected, as a result of logicoperation of said address data; a write control circuit for writinginformation into said selected memory cell transistor by supplying awrite control voltage to said word decoder in response to a write enablesignal such that said write control voltage is supplied to the controlgate electrode of said selected memory cell transistor via said worddecoder; an erase booster circuit for producing an erase voltage, saiderase booster circuit supplying said erase voltage to said word decoderwith a magnitude sufficient to expel the electric charges in thefloating gate of said selected memory cell transistor through the gateinsulation film by tunneling effect; a power supply circuit forsupplying a positive voltage signal to said substrate such that saidpositive voltage signal holds the level of said substrate at a positivevoltage level when erasing information, said power supply circuitproducing said positive voltage signal with a magnitude substantiallyequal to or smaller than the voltage applied to the control gate of saidselected memory cell transistor, said erase voltage thereby beingdivided into said voltage applied to said control gate and said positivevoltage applied to said substrate, wherein said substrate includes afirst well of a first conductivity type opposite to a conductivity typeof said channel region, and further a second well of a secondconductivity type opposite to said first conductivity type and formedinside said first well, said second well including therein a transistorforming said peripheral circuit, and wherein said first and second wellsare reversely biased by applying a bias voltage thereto in an erase modeoperation of the flash-erasable semiconductor memory device, said erasevoltage being applied to said first well via a diffusion region of saidtransistor formed in said first well.
 7. A flash-erasable semiconductormemory device, comprising:a memory cell array including a plurality ofmemory cell transistors, each of said memory cell transistorscomprising:an insulated floating gate provided on a semiconductorsubstrate with a separation therefrom; a gate insulation film providedon an upper major surface of said semiconductor substrate so as toseparate said floating gate from said semiconductor substrate; a channelregion defined in said semiconductor substrate in correspondence to saidfloating gate electrode; a source region and a drain region defined insaid semiconductor substrate at both sides of said floating gate; and acontrol electrode provided on said floating gate with a separationtherefrom by a capacitor insulation film; and a peripheral region, saidperipheral region includinga peripheral circuit; a first well formed insaid substrate in said peripheral region with a conductivity typeopposite to a conductivity type of said channel region; and a secondwell formed inside said first well with a conductivity type opposite tothe conductivity type of said first well, said second well includingtherein a transistor forming said peripheral circuit, wherein said firstand second wells are reversely biased by applying a bias voltage theretoin an erase mode operation of said flash-erasable semiconductor memorydevice, and wherein said erase voltage is applied to said first well viaa diffusion region of said transistor formed in said first well.